Corrosion-resistant heat exchanger

ABSTRACT

A corrosion-resistant, copper-finned heat exchanger for a water heater is provided. The heat exchanger includes a conduit through which water runs, heat-transfer fins extending from the conduit and an anti-corrosive coating containing electroless nickel. The heat-transfer fins contain copper, and the coating is deposited directly onto at least one of the copper heat-transfer fins.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a coiled-heat-exchanger-type waterheater, and more specifically to a corrosion-resistant coating for theheat exchanger coil of that type of water heater. The anti-corrosivecoatings and coating methods described herein are also applicable tolinear-heat-exchanger-type water heaters.

[0002] In known coiled-heat-exchanger-type water heaters, such as aLegend Burkay® Boiler manufactured by A. O. Smith Corporationheadquartered in Milwaukee, Wis., water flows through the heat exchangerwhile hot products of combustion flow over the outside of the heatexchanger. If the water in the heat exchanger is too cold, some of thegases in the products of combustion may reach their dew points andcondense on the heat exchanger. As a result, a condensation ofcorrosive-combustion products may form on the heat exchanger, therebyleading to corrosion of the coil. This, in turn, may causeinefficiencies in, or even failure of (i.e., leaking), the heatexchanger. More particularly, the corrosion products can accumulate onand between heat-transfer or finned surfaces extending from the heatexchanger, thereby resulting in restricted airflow through the heatexchanger. The restricted airflow can cause problems with combustion andalso cause eventual leakage of the heat exchanger.

[0003] One known way to prevent corrosion in the heat exchanger is tocoat the heat exchanger with lead. The typical process for this measureincludes dipping the heat exchanger into a molten pool of lead to obtaincomplete coating of the heat exchanger. This process is typically nolonger used due to the hazards associated with lead.

[0004] Another known way to combat such corrosion is to raise thetemperature of the water being introduced into the heat exchanger toreduce the likelihood of condensation. This is sometimes done by routingor recirculating some of the hot water from the exit of the heatexchanger back to the inlet to mix with the cold water being introduced,thereby raising the temperature of the coil above the dew point. Suchrecirculation systems often require a pump and control system which canadd cost and complexity to the system.

SUMMARY OF THE INVENTION

[0005] The invention provides a copper-finned heat exchanger for a waterheater. The heat exchanger comprises a conduit through which water runs,heat-transfer fins extending from the conduit and an anti-corrosivecoating comprising electroless nickel. The heat-transfer fins are madepreferably of copper, and the coating is deposited directly onto atleast one of the copper heat-transfer fins. In one embodiment of theinvention, the anti-corrosive coating is about 0.05 mils to about 10mils in thickness.

[0006] In addition, the invention provides a water heater. The waterheater comprises a housing, a combustor positioned within the housing, aflue positioned above the combustor in the housing and a copper-coiledheat exchanger positioned within the housing. The heat exchanger has aconduit through which water runs, and heat-transfer fins extendtherefrom. An anti-corrosive coating is chemically deposited directlyonto a portion of the copper heat exchanger, and the anticorrosivecoating preferably includes electroless nickel. The anti-corrosivecoating may be about 0.05 mils to about 10 mils in thickness.

[0007] Furthermore, the invention provides a method of preventingcorrosion of a heat exchanger for a water heater. The method comprisesimmersing a copper heat exchanger into an aqueous-chemical-depositionbath comprising at least one of nickel, cobalt, palladium or platinum.The method further comprises electroless-chemically depositing anelectroless coating selected from the group consisting of nickel,cobalt, palladium, platinum or a combination thereof onto at least aportion of the heat exchanger. The electroless coating preventscorrosion of the heat exchanger when the heat exchanger is used inconjunction with a functioning water heater.

[0008] Other features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdetailed description, claims, and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a perspective view of a water heater and a coiled heatexchanger (shown in phantom) embodying the present invention.

[0010]FIG. 2 is a top plan view of the coiled heat exchanger of thewater heater in FIG. 1.

[0011]FIG. 3 is a side view of the top portion of the coiled heatexchanger in the water heater in FIG. 1.

[0012]FIG. 4 is a partial cross-sectional view taken along line 4-4 ofFIG. 1.

[0013]FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 4.

[0014]FIG. 6 is a perspective view of one of the heat-transfer fins ofFIG. 5.

[0015]FIG. 7 is a greatly expanded cross-sectional view taken along line7-7 of FIG. 4.

[0016] Before one embodiment of the invention is explained in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangements of the componentsset forth in the following description or illustrated in the drawings.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways. Also, it is understood that thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including” and “comprising” and variations thereof herein is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items. The use of “consisting of” and variations thereofherein is meant to encompass only the items listed thereafter. The useof letters to identify elements of a method or process is simply foridentification and is not meant to indicate that the elements should beperformed in a particular order.

DETAILED DESCRIPTION OF THE INVENTION

[0017]FIG. 1 illustrates a water heater 10 including a housing 14, acoiled heat exchanger 18 within the housing, and a combustor 22positioned within the housing. Again, linear-type heat exchangers may beused, but coiled heat exchangers, and more particularly copper-coiledheat exchangers, are preferred. A cold-water inlet 26 extends throughthe housing 14 and communicates with one end of the coiled heatexchanger 18, and a hot-water outlet 30 extends through the housing 14and communicates with the other end of the coiled heat exchanger 18. Agas-fuel supply line 34 communicates with the combustor 22 and providesgas fuel to be mixed with air and burned by the combustor 22. Inoperation, the combustor 22 creates hot products of combustion 38 byburning the air/fuel mixture and the hot products of combustion 38 flowover the coiled heat exchanger 18 to heat the water flowingtherethrough. The hot products of combustion exit the water heater 10through a flue 32. Therefore, cold water can be introduced into thecold-water inlet 26 and be heated as it flows through the coiled heatexchanger 18 such that the water is at a desired temperature as it exitsthrough the hot-water outlet 30.

[0018] FIGS. 2-7 better illustrate the coiled heat exchanger 18, whichincludes a coiled-heat-exchange conduit, tube, or pipe 42 havingheat-transfer fins 46 metallurgically bonded to its outer surface. Aflow space 50 is defined between the fins to accommodate the flow ofproducts of combustion.

[0019] With particular reference to FIGS. 3-7, the tube 42 and fins 46are preferably constructed of a copper to promote heat transfer. Thetube 42 and fins 46 can be constructed of a copper alloy or any othermetallurgical mixture containing copper. Preferably, the fins 46extending from the heat exchange conduit or tube 42 comprise purecopper, although the fins 46 may also comprise different copper alloys.The conduit or tube 42 of the heat exchanger 18 may comprise copper,although a copper alloy is more typical. For example, the copper alloymay comprise zinc oxides and irons. In a preferred embodiment, thecopper fins 46 comprise about 99.95 percent copper with a trace ofphosphorus (material specification ASTM B75), and the copper conduit 42comprises about 84 to about 86 percent copper, about 4 to about 6percent tin, about 4 to about 6 percent zinc, about 4 to about 6 percentlead and some trace amounts of iron (material specification ASTM B62).

[0020] A chemical deposition process is used to coat an anti-corrosiveouter layer 54 onto the copper tube 42 and/or fins 46. Theanti-corrosive outer layer may comprise an electroless nickel, cobalt,palladium, platinum or a combination thereof, although electrolessnickel is most preferred. In other words, cobalt, palladium, platinumand combinations thereof can be used as substitutes for nickel.Alternatively, a poly alloy may be applied to the heat exchanger usingthe methods described herein. The poly alloy may comprise combinationsof nickel, boron or phosphorus and other metals such as cobalt, iron,tungsten, molybdenum and combinations thereof.

[0021] Before chemical disposition takes place, however, a conventionalcleaning process is used to remove dirt and impurities from the exteriorsurface of the copper heat exchanger 18. The cleaning process itself maycomprise a variety of electrical, alkaline and/or acid cleaning steps.The purpose of the cleaning process is to provide a clean,contaminant-free copper surface to which the electroless-nickel coatingcan properly adhere. Optionally, a copper or nickel strike may beemployed to initiate or promote adhesion. Typically, the copper ornickel strike is conducted for approximately 4-5 minutes under 4-5 voltsat a temperature of about 140 to 180 degrees Fahrenheit. The copper ornickel strike provides a very thin layer of copper or nickel, whichinitiates and promotes adhesion.

[0022] Next, the coiled, copper-based heat exchanger 18 is introducedinto an aqueous chemical disposition bath as part of theelectroless-chemical-deposition process. In an alternative embodiment ofthe invention, raw copper or a copper-based alloy may be immersed in thebath, and then later fabricated into the heat exchanger 18. Either way,the coating process provides a uniform coating to the exterior surfacesof the heat exchanger 18. The preferred coating process is anelectroless-chemical-deposition process whereby nickel forms aprotective coating on the copper without the use of a constantelectrical current during the majority of the process. Theelectroless-chemical deposition is different from an electro-depositionprocess, whereby an electrical current is used consistently throughoutthe deposition process. Instead, an initial electrical current is usedonly at the very start of the deposition process in order to facilitatethe initiation of the deposition reaction. Generally, electrical currentof about 6 watts is supplied to the bath for no more than 30 seconds atthe start of the electroless-deposition process. Subsequently, noelectrical current is provided. Although electroless-depositionprocesses are preferred, electroplating methods and vacuum depositionmethods may be used to apply the corrosion-resistant coating.Electroless-deposition is preferred because electroplating techniquesmay clog the space between the tips of the fins of the heat exchangers,may not uniformly distribute the coating on the copper and may alsocreate voids.

[0023] Generally speaking, any aqueous bath comprising nickel ions issuitable for use with the electroless-chemical-deposition methodsdescribed herein. Alternatively, cobalt, palladium and platinum can beused instead of or combined with nickel. As a result, the discussionpertaining to the use of nickel herein also applies to using cobalt,palladium and platinum. Preferably, the bath comprises both nickel andphosphorus, although the presence of phosphorus in the bath is notrequired. Nickel, as well as phosphorus, tend to improve theanti-corrosive characteristics of the resulting coating. In addition,the aqueous solution may also include sodium hypophosphite, an acid aswell as other boron additives or derivatives as discussed below.

[0024] In one embodiment, nickel sulfate provides the requisite nickelions to the solution. Nickel sulfate, and more particularly nickel, isgenerally preferred in a concentration of about 20 to about 100 gramsper liter of solution, and more particularly about 80 to about 90 gramsper liter. Other compounds containing nickel can also be used to supplythe nickel to the bath. Nickel is preferred because it possesses acoefficient of thermal expansion that is similar to that of copper.These two elements are also similarly situated on the periodic table ofelements, and therefore, share similar chemical and physical properties.In addition, nickel has a heat of evaporization that is greater than,but also similar to, copper. More particularly, the heat ofevaporization of copper is about 300.3 kilijoules per mole, while theheat of evaporization of nickel is about 370.4 kilijoules per mole.Because nickel has a greater heat of evaporation, it tends to protectthe copper onto which it is coated. The compatibility of these elementsresults in an anti-corrosive coating that does not inhibit the heattransfer of the copper.

[0025] Sodium hypophosphite is generally preferred in the bath in aconcentration of about 10 to about 40 grams per liter of solution. Morepreferably, the sodium hypophosphite is present in the solution in aconcentration of about 15 to about 20 grams per liter. The greater theamount of phosphorus in the resulting coating, the duller the finalappearance thereof. The intended brightness of the resultingelectroless-nickel coating may dictate the amount of phosphorus to beused in the solution.

[0026] The presence of acid in the bath is also preferred in order tofacilitate chemical deposition. A preferred concentration of the acid isabout 20 to about 40 grams per liter of solution, and more preferablyabout 25 to about 35 grams per liter. One preferred acid is formic acid,although other acids are also suitable for use in the solution.

[0027] In addition, other boron additives or derivatives may be added tothe solution. Examples of boron derivatives include boron hydrate andsodium borohydrite. Generally, residual amounts of boron derivatives arepresent in the bath solution, e.g. concentrations of about 0.3 grams toabout 0.9 grams per liter of solution. The boron additives enhancefinishing, minimize porosity and provide uniformity in the nickelcoating.

[0028] The remainder of the deposition solution is water and impurities.

[0029] The heat exchanger 18 is immersed in this chemical-depositionbath or solution in order to coat the copper exterior of the heatexchanger 18 with the electroless-nickel coating. Except for an initial,brief exposure to electrical current, an electrical current is notintroduced into the bath for the majority of thechemical-deposition-bath process. The initial electrical current is notrequired, but can be used to accelerate the process.

[0030] The temperature at which the bath is kept during the chemicaldeposition process may vary. Preferably the temperature ranges fromabout 80 to about 210 degrees Fahrenheit, although a temperature rangeof about 140 to 210 degrees Fahrenheit is more preferred, and atemperature of about 160 to about 190 degrees Fahrenheit is mostpreferred. The pH of the solution bath is typically maintained in arange of 2.0-14.0, although a range of 3.0-6.0 is most preferred for anacid deposition, while 10.0 to 14.0 is preferred for an alkalinedeposition.

[0031] Length of exposure of the heat exchanger 18 to the bath may alsovary. Exposure to the bath may last from 5 minutes to several hours.Exposure to the solution partially dictates the thickness of theresulting electroless-nickel coating.

[0032] In addition to nickel, the coating may also comprise somephosphorus if phosphorus is present in the deposition solution. In otherwords, a tight-knit nickel-and-phosphorus network may form on thecopper-based exterior of the heat exchanger 18. Typically, thenickel-and-phosphorus network comprises about 0.01 to about 16 percentphosphorus, and more preferably about 6 to about 9 percent phosphorus,and the remainder nickel. Cobalt, palladium and platinum can besubstituted for the nickel in the network. The outer electroless-nickelcoating or nickel-phosphorus network typically has a thickness betweenabout 0.05 mils to about 10 mils. More preferably, the thickness of thecoating is between about 0.1 mil to about 1.5 mils, and most preferablybetween about 0.25 mils and about 1 mils.

[0033] After being exposed to the deposition solution, the heatexchanger 18 is rinsed with water. A chromium seal may also be used toseal each of the remaining reactant sites.

[0034] The corrosion resistance of the present invention providesseveral advantages over known systems. The nickel coating on the copperprovides an excellent combination of corrosion protection and heattransfer. The coating is also environmentally safe and also thermallyconductive. In addition, the coating can withstand the extremetemperatures associated with combustion.

[0035] As discussed above, in other water heaters, the gases ofcombustion reach their dew point and cause a corrosive condensate toform on the heat exchanger. In the water heater of the presentinvention, however, the anti-corrosive coating prevents corrosion.Therefore, cold water can be supplied to the water heater without beingpreheated. As a result, there is no need for a recirculation pump orcontrol system to route hot water back to the cold-water inlet in thepresent invention. By using the electroless coating, and moreparticularly, the electroless nickel coating, cold water can be feddirectly to the boiler, thereby eliminating the external plumbing andcontrol circuit. This, in turn, greatly reduces costs, improves thermalefficiency and greatly simplifies the system. More particularly, theresulting water heater is environmentally friendly because less energyis required due to the elimination of the recirculation step. Theoverall efficiency of the water heater is also greatly enhanced. Inaddition, manufacturing costs are reduced because the extra plumbing andthe control circuit are eliminated.

[0036] Other coatings such as organic-silicone polymers andinorganic-silicone technology as well as sol-gel technology includingcoatings such as epoxy, silicone/epoxy and silicone/acrylic have beentried, but have failed. This is primarily due to insufficienttemperature limits or differences in the coefficient of thermalexpansion between the copper and the coatings. Again, nickel works wellwith copper because they possess similar coefficients of thermalexpansion as well as other chemical properties.

EXAMPLE

[0037] Copper heat exchangers having copper-alloy tubes and essentiallypure-copper fins were coated with an electroless-nickel coating andtested for corrosion as discussed below. The copper fins comprised about99.95 percent copper with a trace of phosphorus (material specificationASTM B75), and the copper tubes comprised about 84 to about 86 percentcopper, about 4 to about 6 percent tin, about 4 to about 6 percent zinc,about 4 to about 6 percent lead and some trace amounts of iron (materialspecification ASTM B62). The copper heat exchangers were cleaned beforebeing exposed to the chemical-deposition baths discussed below.

[0038] Chemical-deposition baths comprising about 84.26 grams of nickelsulfate, about 15.9 grams of sodium hypophosphite, about 27.62 grams offormic acid and about 800 grams of water per liter of solution were usedin the tests. The temperature of the baths was maintained between 160 to190 degrees Fahrenheit at a pH of about 4.4 to 4.6. The copper heatexchangers were then immersed in the bath for about 30 to 45 minutes.The chemical-deposition process yielded coatings having a thicknessbetween 0.25 and 0.75 mils depending on the amount of time each heatexchanger was exposed to the bath.

[0039] The coated heat exchangers were then tested in a laboratory. Morespecifically, the nickel-coated-copper heat exchangers were tested for12 cycles of 1 hour at 1000 degrees Fahrenheit and followed by a coldwater quench. The coated heat exchangers successfully passed this test,and showed reduced signs of green corrosion and rust compared to copperheat exchangers having no protective electroless-nickel coating. Inanother test, the nickel-coated-copper heat exchangers were exposed toabout 4000 hours of a salt spray test. More particularly, ASTMB-117Salisbury testing methodology was followed to test the affects ofcorrosion on the heat exchanger. Again, the heat exchangers exhibitedimproved corrosion resistance.

We claim:
 1. A copper-finned heat exchanger for a water heater, the heatexchanger comprising: a conduit through which water runs; heat-transferfins extending from the conduit, the heat-transfer fins comprisingcopper; and an anti-corrosive coating comprising electroless nickel, thecoating being deposited directly onto at least one of the copperheat-transfer fins.
 2. The heat exchanger of claim 1, wherein theanti-corrosive coating is about 0.05 mils to about 10 mils in thickness.3. The heat exchanger of claim 2, wherein the anti-corrosive coating isabout is 0.1 mil to about 1.5 mils.
 4. The heat exchanger of claim 3,wherein the anti-corrosive coating is about 0.25 to about 1.0 mils inthickness.
 5. The heat exchanger of claim 1, wherein the anti-corrosivecoating is applied directly to at least one of the copper heat-transferfins by an electroless-chemical-deposition process.
 6. The heatexchanger of claim 1, wherein the anti-corrosive coating furthercomprises phosphorus.
 7. The heat exchanger of claim 1, wherein thecoating is thermally conductive and does not hinder heat transfer. 8.The heat exchanger of claim 1, wherein the heat-transfer fins comprisepure copper to enhance the thermal conductivity thereof.
 9. The heatexchanger of claim 1, wherein the conduit comprises a copper alloy andhas an electroless-nickel-coating deposited onto at least a portionthereof.
 10. A water heater comprising: a housing; a combustorpositioned within the housing; a flue positioned above the combustor inthe housing; and a copper-coiled heat exchanger positioned within thehousing, the heat exchanger having a conduit through which water runs,heat-transfer fins extending therefrom and an anti-corrosive coatingchemically deposited directly onto a portion of the copper heatexchanger, the anti-corrosive coating including electroless nickel. 11.The water heater of claim 10, wherein the anti-corrosive coating isabout 0.05 mils to about 10 mils in thickness.
 12. The water heater ofclaim 11, wherein the anti-corrosive coating is about 0.10 mils to about1.50 mils in thickness.
 13. The water heater of claim 12, wherein theanti-corrosive coating is about 0.25 to about 1.0 mils in thickness. 14.The water heater of claim 10, wherein the portion of the heat exchangeronto which the electroless nickel is directly deposited is aheat-transfer fin.
 15. The water heater of claim 14, wherein theheat-transfer fins comprise pure copper in order to enhance thermalconductivity thereof.
 16. The water heater of claim 10, wherein theconduit comprises a copper alloy and the conduit is the portion of theheat exchanger onto which the coating is directly deposited.
 17. Thewater heater of claim 10, wherein the coating further comprisesphosphorus.
 18. A method of preventing corrosion of a copper heatexchanger for a water heater, the method comprising: immersing a copperheat exchanger into an aqueous-chemical-deposition bath comprising atleast one of nickel, cobalt, palladium or platinum; andelectroless-chemically depositing an electroless coating selected fromthe group consisting of nickel, cobalt, palladium, platinum or acombination thereof onto at least a portion of the heat exchanger,whereby the electroless coating prevents corrosion of the heat exchangerwhen the heat exchanger is used in conjunction with a functioning waterheater.
 19. The method of claim 18, wherein the electroless coating isabout 0.05 mils to about 10 mils in thickness.
 20. The method of claim19, wherein the electroless coating is about 0.1 mils to about 1.5 milsin thickness
 21. The method of claim 20, wherein the electroless coatingis about 0.25 to about 1.0 mils in thickness.
 22. The method of claim18, wherein the chemical-deposition bath further comprises phosphorus.23. The method of claim 22, wherein the electroless coating is anelectroless nickel-phosphorus network.
 24. The method of claim 23,wherein the heat exchanger is a copper-coiled heat exchanger havingheat-transfer fins, the heat-transfer fins having the electrolesscoating applied thereon.
 25. The method of claim 24, wherein theelectroless nickel-phosphorus network comprises about 0.01 to about 16percent phosphorus.
 26. The method of claim 25, wherein the electrolessnickel-phosphorus network comprises about 6 to about 9 percentphosphorus.
 27. The method of claim 18, wherein the chemical-depositionbath further comprises sodium hypophosphite, an acid, a boron derivativeand water.
 28. The method of claim 27, wherein the bath comprises about20 to about 100 grams of nickel per liter of solution, about 10 to 40grams of sodium hypophosphite per liter of solution, and about 20 toabout 40 grams of acid per liter of solution.
 29. The method of claim28, wherein the bath comprises about 80 to about 90 grams of nickel perliter of solution, about 15 to about 20 grams of sodium hypophosphiteper liter of solution and about 25 to about 35 grams of acid per literof solution.
 30. The method of claim 18, whereby no electrical currentis used during the chemical deposition process.
 31. The method of claim18, whereby an electrical current is used initially after the heatexchanger is immersed in the bath, but for no more than thirty seconds.32. The method of claim 18, whereby the electroless coating canwithstand high temperatures associated with products of combustion. 33.The method of claim 18, wherein the electroless coating comprisesnickel, boron or phosphorus and at least one other metal selected fromthe group consisting of cobalt, iron, tungsten and molybdenum.